It was only when Maddie Harris returned home that the full weight of her experiences hit her.

In the summer of 2015, Harris had been working on the UK festival circuit. As the media began flooding the world with photos of the refugee crisis taking place in Europe, Harris was increasingly uneasy. That September, as the festival season drew to a close, the 31-year-old freelance events manager made a last-minute decision to join a group of friends travelling to northern France to help out at refugee camps. “The intention was to be there for four or five days,” she says. Harris ended up staying for 12 months.

After a couple of weeks working in Calais, Harris heard about a smaller camp, 20 miles away in Dunkirk, that was in a bad way. “I got there and was taken aback,” she says. Around 300 people, including children, were living in flimsy summer tents. Harris took on an unofficial camp management role. Working with a handful of other volunteers, she distributed aid, tried to establish basic facilities and dealt with medical emergencies.

Harris had never done humanitarian work before, so what she didn’t know, she learned. Most mornings she would arrive at the camp at 7.30am and work until 10pm. Often she would get back to her accommodation and stay up late into the night working. The severity of the situation in France meant that any thoughts she’d had about returning to the UK were abandoned. Harris covered her own expenses, using the savings she had been putting aside for a deposit on a house.

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There were soon signs that things were becoming too much for her. By November, the camp population had risen to more than 1,500. The following month, several days of rain left the camp flooded. “That was the point at which I felt like the responsibility for sorting it out was on me,” says Harris. “I couldn’t sleep. I sat up smoking cigarettes trying to answer emails and going to the camp the next day. That’s the point at which the exhaustion just reached the next level.”

In March 2016, in large part down to the efforts of Harris and other volunteers, the authorities in Dunkirk worked with Médecins Sans Frontières (MSF) to open an official camp with wooden cabins and proper infrastructure. (The camp burnt down in April this year.) Three months later, Harris decided her help was needed lsewhere. She had heard reports of the deteriorating situation in northern Greece and decided to move on. In September that year, during what was supposed to be a brief trip back to the UK, she broke down.

"I couldn’t sleep. I sat up smoking cigarettes, and trying to answer emails."

“I was overwhelmed by the thoughts and images in my mind,” she says. “I was crying, having a panic attack. My housemate said, ‘You can’t go back’. I thought I would stay for one more week but I was racked with vomit-inducing anxiety. Eventually I thought, ‘You know what, I do just need to stay here’. So I did. And I went to ground for three months.”

Harris is one of thousands of volunteers who have found themselves on the frontlines of Europe’s migration crisis. As the influx of refugees reached epic proportions in the summer of 2015, ordinary people from all over the world travelled to crisis hotspots to take on roles as lifeguards, medics and support workers. Many had no previous experience of aid work. This grassroots response to a humanitarian crisis was unprecedented in modern times. There is no doubt it saved countless lives. But there are also signs it has taken a huge psychological toll.

From a mental health perspective, aid work is a high-risk profession. A 2015 survey by the Guardian found nearly four in five aid workers had experienced mental health issues. Research has suggested that up to 30% of humanitarian workers are affected by post-traumatic stress disorder.

Most large NGOs take steps to mitigate these risks. “There’s no one who doesn’t experience some form of stress, or feel overworked or overwhelmed,” says Jose Hulsenbek, head of HR at MSF. Staff are trained in stress management and encouraged to speak to counsellors if they feel vulnerable. Aid workers are debriefed by a psychologist when they return from assignments.

But even with these measures in place, mental health problems are still a risk. Most independent volunteers receive nothing like this level of support. Toni Brodelle is a wellbeing consultant who has offered her services to around 300 refugees and volunteers to help them cope with trauma. Asked about the scale of the mental health problem among volunteers, she says: “It’s huge. I would be hard-pushed to find someone who hasn’t been significantly shaped or changed.”

Burnout is a commonly used term in the volunteer community, referring to the mental and physical exhaustion that sets in after months spent working in the field. In Dunkirk, Harris would work 18-hour days for weeks without a break. Frequently, it is only when volunteers return home that the impact of their work becomes clear.

Some of the 1000s of life jackets from refugees crossing the Aegean sea last year, now in a dump in Greece. Photograph: Aris Messinis/AFP/Getty Images

Skylar, a 26-year-old graduate student from Massachusetts who asked only to be identified by her first name, spent three months working with unaccompanied children on the Greek island of Lesvos earlier this year. Early on in the trip, she remembers a young boy opening up to her about his experiences. “I went home to my apartment and broke down into tears,” she says. “You just get into survival mode. And so I didn’t really process any of it until I got home. And now, being home, I realise how much it has affected me mentally, emotionally, spiritually.”

Skylar is still struggling to adjust. “I exercise, do yoga, hang out with friends, but there’s an emptiness to it because in the back of my mind I’m thinking about the boys and what they have been through,” she says. “I’m stuck between these two worlds and nobody else understands. It’s hard to move on. I feel guilt, for one, like I’ve abandoned them. So there’s a sense of guilt and abandonment, and of loneliness.”

Among the volunteers who shared their experiences for this article, guilt and loneliness were recurring themes. Many were unwilling to recognise their problems while the people they were trying to help were suffering so much. One describes thinking: “These people have travelled from Syria or Afghanistan, they have seen relatives slaughtered. How dare I be mentally ill?” Others describe having no one to speak to who could relate to their experiences. “When you’re there, you’re part of a community that either understands what is going on or is as angry as you are,” says one volunteer. “When I came home, that wasn’t there. People don’t know what to say.”

A sense of helplessness is also common. Jenny Massey is a 40-year-old volunteer from Edinburgh who has travelled to the Greek island of Chios three times in the past 12 months. “When you’re there, every single minute of your day you’re doing something important for people,” she says. “When you get back, every minute of every day is inconsequential. And now you’re aware of what’s happening in the world and there’s nothing you can do.” Given the prevalence of these feelings, it’s perhaps no surprise that many volunteers don’t see giving up as an option.

After three months off, Harris has recovered sufficiently to get back to work. She has since returned to northern France and is setting up a network of witnesses to record human rights abuses across Europe. “This is it,” she says. “Once you know, how could you possibly not do something?” This time, she’ll be taking steps to avoid burnout, or worse. However, she’s also aware there will always be risks. “It feels like a small price to pay,” she says.